It is God's purpose and plan for us all to grow old and to go through physical changes. But it is not God's purpose for us to grow old and stop serving Him.
Everything changes because our God is a God of change. There is nothing stationary about God. He is always moving and progressing forward and, as Christians, we are part of something God is doing, regardless of our age or physical condition. So regardless of how old you are or how you feel physically, God still has something for you to do for His kingdom. Regardless of age, you have high value to God.
God does not want his people to retire from His service. He needs you now more than he has ever needed you in all your life. He needs you during all of your living years. I don't see any place in God's concept for life that people retire and sit in a rocking chair all the rest of their days. That is not God's plan for your life. Stop living like you are already dead!
Be excited about your future regardless of how long your past goes back. The doors ahead of you are open. Be creative and productive all the way up to the time God calls you home. Get involved in your church or community organization. Be a light to the world and shine until your last breath. Don't let your age or physical inabilities ever stop you from doing what God is asking you to do. Grab hold of your eternal vision and God will use you for that ministry He still has planned for you.